The present description relates to diagnosis for a multiple cylinder engine, more particularly to a method and system to detect imbalance of air fuel ratio between the cylinders of a multiple cylinder engine.
It may be desirable to have equivalent air-fuel mixtures in engine cylinders so that engine emissions can be controlled. For example, when similar air-fuel mixtures are combusted in each engine cylinder, the cylinder air-fuel mixtures may be adjusted to improve efficiency of a catalyst in the exhaust system with a reduced possibility of producing a misfire. In particular, when all engine cylinders are combusting substantially equivalent air-fuel mixtures, the engine air-fuel mixture may be leaned to improve catalyst efficiency with less possibility of a single cylinder misfiring due to that one cylinder reaching a lean combustion limit before other cylinders. On the other hand, if one engine cylinder is operating at a leaner air-fuel mixture than other cylinders, then the possibility of one cylinder misfiring during leaning of engine air-fuel ratio may increase. It is therefore desirable to provide a method for improving the detection of cylinder air-fuel imbalance.